A purge dam forms a barrier, within either a pipe or a tube, to form, in cooperation with a second purge dam, each flanking the proposed weld site, thereby enclosing a small volume of the interior of the pipe or tube. Prior to welding, an inert gas is used to “purge” or eliminate oxygen from the interior at the weld site. Pressing oxygen out of the enclosed volume assures that metal heated to a molten state will not oxidize in the presence of free oxygen, thereby preventing any compromise to the integrity of the weld.
As a matter of nomenclature throughout this application, tubes and pipes are different; while this invention applies with equal vigor to each of tubes and pipes, it is the understanding of that the proper nomenclature is selected by application. Generally, pipes accommodate larger applications with sizes that range from a half-inch to several feet. Tubes, on the other hand, are generally used in applications that require smaller diameters. Tubes are often put to use in applications that require precise outside diameters, like with cooler tubes, heat exchanger tubes and boiler tubes. Pipes have a pressure rating and are schedule, which is why they are often used to carry fluids that must be contained. Without exclusion of pipes, the term “tubes” will be used throughout to refer to both of pipes and tubes. No prejudice can be presumed as the inventive purge dam assembly can be applied with equal facility to each of tubes and pipes and the selection of “pipes” is merely to make the claims definite in light of this very similar nomenclature. For purposes of this application, there is no intended distinction between the two terms and in the context of this application the term “pipes” encompasses both, even acknowledging that in the industry, there is a distinction.
The volume confined between the two purge dams is generally kept small to assure the rapid elimination of oxygen behind the weld. Purging by introducing a volume of an inert gas to displace oxygen is a dilution phenomenon. Purge time and the volume of purge gas required are each proportionate to the volume being purged; naturally, the larger the enclosed volume, the greater the volume necessary to displace the oxygen. Acceptable oxygen concentrations vary by industry and application. For example, in one market requiring very high quality welding, the semiconductor and pharmaceutical industry, an oxygen concentration in the volume between the purge dam is required to be as low as approximately one part per million (ppm).
There are several types of purge dams on the market but very few are used in either of the semiconductor and pharmaceutical industry markets where, for example, electropolishing is necessary to remove surface imperfections because such imperfections enhance the risk of contamination. Because conventional purge dams are made of, for example, water soluble paper, the rinsing step used to remove those dams nearly assures the presence of unwanted remnant or residue.
Fastidious and scrupulous care is necessary to exclude sources of contaminants because the tubing system will eventually be conveying gases requiring purity having less than a part per billion or even a part per trillion (ppb or ppt) of contaminants. Even among those other conventional types of purge dams, apart from those formed of soluble paper, such as inflatable cloth dams coated with elastomer, inflatable elastomer dams, or rigid disks with peripheral elastomeric seals, the interior surface of the pipes might suffer some scratching or other disfigurement resulting from the outward pressure the elastomer exerts when sealing. Most elastomers, because of the cost of the pure elastomer resin also contain hard fillers such as carbon or glass that can, also, scratch surfaces. Semiconductor industry standards prevent the use of any of the conventional purge dams; all of the conventional purge dams would either degrade the electropolished inner pipe surface by creating fine scratches or leave dilute residues from the elastomers.
SEMI (Semiconductor Equipment and Materials International) standards provide a general outline for the tubing (material, manufacturing standards), surface properties (cleanliness, roughness), welding methods and quality control, and post weld inspection criteria used by that industry. For example, in the 1995 expression of the grading for surface roughness there are three standards, which for the purposes of explanation will be ranked good, better, and best. For good surfaces, SEMI standards require that no scratch may exceed a depth of greater than 0.3 micrometers, for better, 0.25, and for best 0.18. The averages for each standard with multiple measurements are, for good, 0.25; for better, 0.18, and for best, 0.13. Use of conventional purge dams makes welding product to these specifications impossible. Surface scarring in excess of allowable standards results from elastomer pressure on the interior surface of the pipe.
To help in the understanding of the scale of these defects, consider that a human hair is about 75-100 microns or, alternatively, micrometers, in diameter. In sufficient numbers particles 200 times smaller (0.5 micron) than the human hair can cause major disaster in a cleanroom. Contamination can lead to expensive downtime and increased production costs. In fact, the billion-dollar NASA Hubble Space Telescope was damaged and did not perform as designed because of a particle smaller than 0.5 microns. Elastomers used by conventional means to form purge dams regularly contain particulate far larger in size than the 0.5 microns described herein.
What is lacking in the conventional art is a method and purge dam that can be used without contact between elastomers which might abrade or otherwise scar interior surfaces of pipes. SEMI standards, for one, will not allow conventional means for damming a pipe as the resultant interior surface would be put into a state of roughness that exceeds the standard.